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Affected System Operator Studies

In coordination with Eversource, ISO-New England has determined transmission studies are needed for many DG projects interconnecting to Eversource's infrastructure.

The Affected System Operator (ASO) studies seek to identify whether a distributed generation (DG)  project impacts the transmission system and/or the electric power systems of neighboring electric distribution companies, and the extent transmission system modifications are required to maintain the safe and reliable operation of the region's transmission system and distribution infrastructure.

If modifications are needed, DG projects could share in the cost of that work.

The transmission analysis and studies are being performed by Eversource’s transmission staff, consulting with other affected systems operators, including ISO-New England (ISO-NE). Results of the study need to be approved by ISO-NE.

Analysis requirements

Based on factors such as the size of a generator, operating voltage level and connection with a transmission line, various levels of analysis may be required. Eversource works with ISO-NE to determine the initial level of transmission study needed for each project.

Our goal is to safely and expediently get distributed generation projects up and running as quickly as possible. We are committed to transparency and will work with potential project stakeholders to answer questions and expedite the study process.

As part of D.P.U. 19-55, Eversource and the other Electric Distribution Companies prepared a diagram illustrating the process and decision points for the initial ASO Impact Screen (PDF). This process was created with stakeholder input, including ISO-NE.

Additional information about ASO studies is available on the Department of Public Utilities website under docket 19-55.

DG Guidelines for Interconnection

We have established Distributed Generation Guidelines for Interconnection in collaboration with the Massachusetts Department of Public Utilities (MA DPU). This document:

  • Explains the electric distribution company's (EDC) process for screening projects in ASO Impact Screens throughout the application lifecycle
  • Details reporting, communications and updates to affected customers during the application process and throughout any ASO study moving forward

Technical Data Requirements

Current Phase

The 2025 Level 0 ASO studies in Western Massachusetts and Greater Boston have been completed. The projects included in these Level 0 studies have received Proposed Plan Application (PPA) Approvals from ISO-NE in 2025.

The 2025 Level 3 ASO studies are underway. The projects currently in the Level 3 ASO studies are expected to receive Section I.3.9 PPA approval from ISO-NE by December 31, 2025, provided no unforeseen delays that are beyond Eversource’s control. As ISO-NE’s presentation to the NEPOOL Transmission Committee on April 17, 2025 states “As long as the ASO studies are within 90 days of achieving I.3.9 approval (and all modeling information is provided) by the beginning of the Transitional Cluster Study (i.e. by October 10, 2025), the ASO studies can complete.”

View the list of ASO projects to be approved by October 30, 2025 (PDF)

Initial Determination Requests

When an ASO Impact Screen results in the potential need for a DG project to undergo an ASO study, we will submit the project’s data to ISO-New England (ISO-NE) with a request for determination on being included in an ASO study.

ASO Maps

View our comprehensive hosting capacity maps for Eastern and Western Massachusetts, including the five ASO study areas: Springfield, Pittsfield, Greater Boston, Upper Southeastern Massachusetts (Upper SEMA) and Cape.

These maps reflect the current status of the ASO studies:

Notifications, FAQs and Important Information

As required by the Department of Public Utilities (DPU), Eversource has reached out via email to developers with projects needing an ASO study. PDFs of the notification are available below. Information shared during the ASO Customer Kickoff Meeting is also linked below in PDF format.

2020 ASO (announced 10/24/19)

Notices and information

All files are in PDF format.

Reports to impacted customers

Every two weeks prior to the study commencing, Eversource reported out to impacted customers. An archive of those reports is available below.

On a monthly basis, Eversource will report out on the status and updates of the ASO studies that we have commenced, until those studies are completed. There is currently one group of ASO customers receiving these monthly reports (customers who are in the Level 3 ASO study announced on October 24, 2019). Previously Level 0/1 ASO study customers and customers who had an interconnection service agreement (ISA) were receiving these reports as well. An archive of those reports is available below.

2021 ASO

Notices and information

All files are in PDF format unless otherwise noted.

Reports to impacted customers

Monthly (PDFs)

Biweekly (PDFs)

Every two weeks prior to a study commencing, Eversource will report out to impacted customers.


2023 ASO

Notices and information

All files are in PDF format unless otherwise noted.

Western MA and Greater Boston

Reports to impacted customers

Monthly (PDFs)

Biweekly (PDFs)

Every two weeks prior to a study commencing, we will report out to impacted customers.

2024 ASO

Notices and information

All files are in PDF format unless otherwise noted.

Western MA and Greater Boston

Reports to impacted customers

Monthly (PDFs)

Biweekly (PDFs)

Every two weeks prior to a study commencing, we will report out to impacted customers.

Have questions?

If you have questions or would like more information about the ASO study process, please email your account executive.